1. Field of the Invention
The present invention relates to a method of controlling the running state of a vehicle by controlling friction force between the surface of a road and tires and to an apparatus for carrying out the method.
2. Description of the Prior Art
Means of controlling the running state of an automobile include a driver's empirical driving technique and an electronic control system such as ABS (Anti-Lock Brake System) for controlling the speed of each wheel and a slip ratio (10 to 30%) to obtain a high friction coefficient or traction control system for controlling a brake unit, engine speed or torque.
The above electronic control system controls to exhibit the friction force of each tire to the maximum in order to stabilize a vehicle. This system aims to ensure vehicle directionability, steerability and controllability so as to avoid a danger by controlling the locked state of wheels by increasing or decreasing the hydraulic pressure of a brake or the torque of a driving system to obtain a slip ratio that ensures fully great cornering force and great braking force. Therefore, the slip ratio-friction characteristic curve of the tire, as shown in FIG. 8, is estimated and the slip ratio is controlled based on this estimated curve to obtain the maximum friction coefficient in limits not to cause a side slip. In order to detect the running state of the vehicle, an acceleration sensor and a yaw rate sensor are mounted on the vehicle to control the slip ratio of the tire based on signals from these sensors to stabilize the vehicle.
However, the slip ratio-friction characteristic curve of the tire greatly differs according to the condition of a road surface such as dry, wet, snowed or iced surface as shown in FIG. 8. Therefore, the slip ratio-friction characteristic curve used for the above ABS control may differ from a slip ratio-friction characteristic curve under the condition of a road surface at the time of actual control and it is difficult to estimate the accurate speed of the vehicle at the time of cornering, thereby reducing the reliability of the slip ratio. As a result, satisfactory control may not be carried out.
Further, in the above electronic control system, since the above acceleration sensor and the yaw rate sensor are mounted on the vehicle having a lower response frequency than the response frequency of the tire, the feedback control of the electronic control system tends to delay in comparison with a sensing-feedback system around the tires. Since feedback control is carried out after the vehicle having a great inertia mass shows an unstable change, there is a tendency that the vehicle can be stabilized by controlling with a intense feedback signal.
It is an object of the present invention which has been made in view of the above problems of the prior art to improve the running performance of a vehicle and enhance the safety of the vehicle by changing the slip ratio-friction characteristics of each tire by detecting the unstable state of the tire.